This application is a renewal of a previous R01 Cell-of-Origin Effects on Development of Colon Cancer. During this granting period, we discovered 1) the pan-ErbB inhibitor, Lrig1, marks largely quiescent colonic epithelial stem cells that are distinct from Lgr5 and 2) Lrig1 acts as a tumor suppressor in vivo. We found that tamoxifen- induced loss of one Apc allele in Lrig1CreERT2;Apcfl/+ mice results in multiple, highly dysplastic colonic adenomas that exhibit stochastic loss of the 2nd Apc allele. An identical approach in Lgr5 driver mice results in no tumors. We have shown that the Lrig1-driven tumors are more histologically advanced than ApcMin colonic tumors, and present preliminary data that the Lrig1-driven tumors exhibit an RNASeq profile similar to that of human colorectal cancer. These findings have led us to focus on Lrig1-related tumors in this retitled application, Roles for Lrig1 in Intestinal Neoplasia. Based on the importance of Egfr in mediating the highly penetrant duodenal tumor phenotype observed in Lrig1-null mice and the marked upregulation of Egfr ligands in colonic tumors of Lrig1CreERT2;Apcfl/+ mice, we have made a new Egfr reporter mouse (EgfrEmGFP) to monitor the Lrig1/Egfr relationship. We also have generated an antibody to mouse Lrig1 that recognizes non-glycosylated Lrig1 that stains long-lived, lineage-labeled, quiescent Lrig1 stem cells and staining is increased in Lrig1 stem cell-driven intestinal tumors. For the first time, we have access to an Egfr antibody that effectively blocks mouse Egfr. This will allow us to examine the mechanism by which Egfr promotes the establishment of colonic neoplasia, as well as to determine the therapeutic efficacy of Egfr blockade in treating established tumors. Given the increasing biological importance of Lrig1, we also propose experiments to examine selected aspects of the regulation and function of LRIG1. Initially, we will determine basolateral sorting elements in the cytoplasmic tail of LRIG1 and the impact of glycosylation on LRIG1 function. At the outset, we will examine these events in the context of EGFR activity, but, as the grant proceeds, we will be alert to additional non-EGFR effects of LRIG1.